Sailing Zephyr IVvideo
Volvo Penta 2003 Mystery Solved With This Simple Check!
8 November 2025 · 5 min · 3K views · Gosport
What this video covers
A Volvo Penta 2003 engine aboard Zephyr IV repeatedly overflowed its freshwater cooling header tank, leaving coolant traces down the engine door and in the bilge.0:14 Dave, based at Gosport and Warsash, initially suspected a failed heat exchanger — the coolant had turned dirty brown, consistent with seawater intrusion — and replaced it with a second-hand spare sourced from eBay.2:03 After the swap, the overflow continued: with plain water substituted for coolant, Dave watched a thin stream return to the header tank, then a sudden violent surge resembling boiling.2:53 Internet research pointed to the thermostat bypass nozzle, a small rubber-pipe nipple that routes coolant back to the header tank while the engine warms up.3:32 The hole had become almost completely blocked.4:29 A cocktail stick could not clear it; Dave drilled it out.4:27 The engine ran normally afterward.4:02 The likely mechanism: the coolant pump builds pressure against the blocked nozzle until it overcomes the pump valve, releasing a sudden back-flush into the header tank.4:42 The heat exchanger replacement was unnecessary.5:05 On a Volvo Penta 2003 with an overflowing header tank, check and clear the thermostat bypass nozzle before replacing the heat exchanger.4:55
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